Magnetic resonance imaging (MRI) is commonly used to image internal tissues of a subject. MRI is typically performed by placing the subject to be imaged at or near the isocenter of a strong, uniform magnetic field known as the main magnetic field B0. The main magnetic field B0 results in magnetization of the atomic nuclei that are aligned with the main magnetic field B0.
Thereafter, a radio frequency (RF) pulse is applied to the subject. The RF pulse can cause the magnetization of the atomic nuclei to nutate away from the direction of the main magnetic field B0. After the RF pulse is switched off, the magnetization of the atomic nuclei precesses about the main magnetic field B0 while returning to an equilibrium state (i.e., to be aligned with the main magnetic field B0), causing emission of RF radiation at a characteristic frequency. The emitted RF radiation can be detected and analyzed to yield information that can be used to produce an image of the subject.
To form an image, magnetic field gradients are also applied to the subject in addition to the main magnetic field B0 and the RF radiation. The magnetic field gradients are typically applied along one or more orthogonal axes (x, y, z), the z-axis usually being aligned with the main magnetic field B0, and introduce spatially distributed variations in frequency or phase of the precessing nuclear spins. By applying the RF radiation and the magnetic field gradients in carefully devised pulses and/or sequences of pulses that are switched on and off, the RF radiation emitted can be encoded with spatial information that, when detected and analyzed, can be used to construct detailed images of the subject or object. Various techniques utilizing both specific pulse sequences and advanced data analysis and imaging methods have been developed, providing new advances, as well as introducing new challenges.
An example of a pulse sequence is echo planar imaging (EPI). FIG. 1A illustrates an example of an EPI pulse sequence 100 utilizing a single RF pulse 102. As shown, the RF pulse 102 is applied to an object in an MRI system, resulting in transverse magnetization 104 of the atomic nuclei that decays over time. During decay of the transverse magnetization 104, a readout gradient 106 is applied to the object. As shown in FIG. 1A, the readout gradient 106 includes a pre-phasing lobe followed by a number of readout lobes with alternating polarity. The readout gradient 106 can be applied to the object along a first axis, such as, for example, the x-axis.
Additionally, a phase-encoding gradient 108 is applied to the object. As shown, the phase-encoding gradient 108 includes a pre-phasing lobe followed by a number of blips, with each of the blips having the same polarity and equal area. The phase-encoding gradient 108 can alternatively include a pre-phasing lobe and a small constant gradient concurrent with a train of readout gradient lobes 106. The phase encoding gradient 108 can be applied to the object along a second axis, such as, for example, the y-axis, which is orthogonal to the first axis.
While the readout gradient 106 and the phase-encoding gradient 108 are applied to the object, RF radiation emitted by the object can be sampled according to the acquisition 110. The sampled RF emission can be used to generate k-space data, which can be further processed to produce an image.
FIG. 1B illustrates k-space data 112 corresponding to the EPI pulse sequence 100 shown in FIG. 1A. In FIG. 1B, the kx axis of k-space corresponds to the axis along which the readout gradient 106 was applied, while the ky axis of k-space corresponds to the axis along which the phase-encoding gradient 108 was applied. The bottom horizontal line of k-space data corresponds to the first readout lobe of the readout gradient 106 shown in FIG. 1A. As shown in FIG. 1B, the bottom horizontal line points towards the positive kx values because the first readout lobe, shown in FIG. 1A, has a positive polarity. The first vertical line at the lower-right corner of FIG. 1B corresponds to the first blip of the phase-encoding gradient 108 and points towards the positive ky values because the first blip has a positive polarity, as shown in FIG. 1A. The next horizontal line of k-space data corresponds to the second readout lobe of the readout gradient 106 and points towards the negative kx values because the second readout lobe has a negative polarity, as shown in FIG. 1A. The additional k-space data corresponds to the readout gradient 106 and the phase-encoding gradient 108 in a similar manner. In implementations in which the phase-encoding gradient 108 includes a pre-phasing lobe and a small constant gradient concurrent with the train of readout gradient lobes, the vertical lines of k-space data cannot be present, resulting in a “zig-zag” of k-space data, attributable to the readout gradient 106 together with the small constant phase-encoding gradient.
The entire k-space data can be acquired using only a single EPI pulse sequence shown in FIG. 1. To achieve a high spatial resolution, however, the EPI pulse sequence is typically repeated multiples times, each sampling a subset of k-space data. Multiple subsets of k-space data are combined, for example, in an interleaved fashion, to accomplish the entire k-space sampling needed to form an image. The method of acquiring multiple subsets of k-space data using EPI is referred to in this disclosure as “multi-shot EPI.” Multi-shot EPI can be sensitive to motion of an object in an MRI system, a characteristic associated with artifacts when the method is applied to, for example, a patient. For instance, if the object moves in-between the acquisition of each subset of k-space data, position inconsistency can lead to motion artifacts, potentially degrading image quality. An option for reducing motion artifacts is to arrange the subsets of k-space data as a set of rotating “blades”, resulting in over-sampling of a central region of k-space 112. The central region of k-space 112 can be, for example, a circular region that radially extends from the intersection of the kx-axis and the ky-axis.
To this end, a technique called Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) can be used. According to the PROPELLER technique, k-space data can be acquired in a number of intersecting “blades.” FIG. 1C illustrates an example of a blade 114 of k-space data. As shown, the blade 114 includes a number Nro of samplings in a direction of the readout gradient 106 in this example, along the kx axis as well as a number Npe of lines in a direction of the phase-encoding gradient in this example, along the ky axis.
Each blade of k-space data can be acquired using an EPI pulse sequence, such as that shown in FIG. 1A. FIG. 1D illustrates three blades 1161, 1162, and 1163 of k-space data. The first blade 1161 is offset from the direction of the readout gradient 106 in this example, along the positive direction of the kx axis—by an angle θ1, which is not shown in FIG. 1D. The angle θ1 can be, but need not be, 0°. Further, as shown in FIG. 1D, the second blade 1162 is offset from the positive direction of the kx axis by an angle θ2. Still further, as shown in FIG. 1D, the third blade 1163 is offset from the positive direction of the kx axis by an angle θ3.
As shown in FIG. 1D, the blades 1161, 1162, and 1163 intersect in a central region of k-space, allowing for an over-sampling of the central region of k-space. This over-sampling can be used to reduce motion artifacts and, in turn, improve the quality of images of the object.
While the EPI-PROPELLER technique can reduce or eliminate motion artifacts, EPI-PROPELLER can have other shortcomings. For example, EPI-PROPELLER is sensitive to eddy currents, which can result from the rapidly switching polarity of the readout gradient. Eddy currents typically result in spatially constant phase errors and spatially linear phase errors, which appear in images as so-called constant Nyquist ghosts and linear Nyquist ghosts, respectively. In EPI, constant and linear phase errors can be corrected by acquiring reference scans and deriving the constant and linear phase errors from the reference scans to be used for phase correction. However, in EPI-PROPELLER, the constant and linear phase errors must be corrected on a blade-by-blade basis. Thus, in EPI-PROPELLER, the constant and linear phase errors are typically corrected by acquiring reference scans for each blade, and then deriving the constant and linear phase errors for each blade from the reference scans. This process can be time-consuming and inefficient, and can be increasingly time-consuming and inefficient as the number of blades increases.
EPI-PROPELLER can also suffer from oblique phase errors resulting from anisotropy among the gradient axes. In particular, for oblique blades (for example, θ2 1162, which does not equal 0), the readout gradient and the phase-encoding gradient are produced by combination of multiple physical gradient axes. In these cases, gradient anisotropy among the physical gradient axes can lead to inconsistent k-space shifts along the phase-encoding direction. The shifts are known as oblique phase errors, which result in so-called oblique Nyquist ghosts (ONG). For an example of a technique of reducing oblique Nyquist ghosts, please refer to U.S. Pat. No. 5,672,969 to Zhou et al., titled “Reduction of Nyquist ghost artifacts in oblique echo planar imaging.” As with the constant and linear phase errors, the oblique phase errors in EPI-PROPELLER techniques must also be corrected for each blade, typically by measuring the oblique phase errors on a blade-by-blade basis, which can be time-consuming and inefficient.
The art is thus in need of methods and systems for correcting constant, linear, and oblique phase errors in EPI-PROPELLER to efficiently produce MRI images containing fewer artifacts.